q
Can I update Plesk Onyx Preview to the RTM version?

It is technically possible, but we do not officially support this upgrade path and thus cannot guarantee that everything will work fine.

q
When will Plesk Onyx support Windows Server 2016?

Windows Server 2016 is currently in the "development preview" stage, and is scheduled to be released in Q4 2016. It is possible to install Plesk Onyx on a Windows Server 2016, however, we do not guarantee stable operation. Official support will be announced once Microsoft releases Windows Server 2016.

q
Is it possible to secure Plesk with Let's Encrypt?

Yes, it is possible. You need to install the Let's Encrypt extension and create a domain in Plesk with a name matching your server's hostname (as every SSL certificate is issued for a specific hostname). You can also secure Plesk with the Plesk Security Advisor extension.